A subject of the present invention is an electrode suitable to be used for the electrolysis of water in a liquid electrolyte medium, a method for formulating such an electrode, a device for the electrolysis of water comprising said electrode, and a method for producing a hydrogen/oxygen mixture or hydrogen alone or oxygen alone.
Pure oxygen is mainly used in the iron and steel industry and in petrochemistry. Other methods also require large tonnages of oxygen, in particular the bleaching of papermaking pulp, the reprocessing of certain types of chemical waste and the production of high-temperature flames. Oxygen is also used as a medical gas in normobaric or hyperbaric oxygen therapy.
Currently, oxygen is obtained industrially mainly by cryogenic separation of the compounds of air, i.e. by liquefaction of air followed by fractional distillation. Pure oxygen can also be obtained by the electrolysis of water.
Hydrogen is one of the raw materials of the chemical and petrochemical industry. It is used in particular for the production of ammonia and methanol and for oil refining; it is also utilized in the metallurgy, electronics and pharmacology sectors, as well as in the processing of foodstuffs.
Hydrogen is also a promising energy carrier, as a replacement for fossil hydrocarbons, in particular in the field of transport. It can be used directly in internal combustion engines, or it can supply fuel cells producing electricity. It is also an energy storage means which can be used in case of need in installations for the production of electricity of the wind power, and photovoltaic panel type and nuclear power stations where the electricity generated is not constant.
Hydrogen is not directly available in nature but it can be produced from three major sources which are fossil, nuclear and renewable energy sources.
Today, 90% of hydrogen gas is produced industrially either by steam reforming of methane (cracking of natural gas using high-temperature steam) or by partial oxidation (production of hydrogen from heavy hydrocarbons and oxygen). These two methods have the drawback of emitting large quantities of CO2.
A third method, electrolysis of water, constitutes the most “sustainable” solution for the production of hydrogen. It is a clean means of producing hydrogen because the greenhouse gas (GHG) and CO2 emissions per kilogram of hydrogen produced are essentially linked to the electrical energy source used for the electrolysis. This means of producing hydrogen can be supplied with electrical energy of renewable origin and makes it possible to store electricity in chemical form.
Electrolysis of water consists of breaking down the atoms of oxygen and hydrogen combined in the water molecules, according to the reactionH2O→H2+1/2O2 
An electrolysis cell is constituted by two electrodes (anode and cathode, electronic conductors) connected to a DC generator, and separated by an electrolyte (ion-conducting medium).
This electrolyte can be either:                solid, and can then be:        either a proton exchange polymer membrane: in this technology, called PEM (Proton Exchange Membrane), a proton exchange membrane or polymer electrolyte membrane is used. This is a semipermeable membrane that allows proton conduction while being impermeable to gases such as oxygen or hydrogen. The advantages of PEM technology are the compactness, the simplicity of operation and the limitation of corrosion problems. However, the cost of the polymer membrane and the use of catalysts based on noble metals lead to relatively expensive equipment.        or an O2− ion-conducting ceramic membrane: one of the features of this technology (SOFC—Solid Oxide Fuel Cell) is the use of a solid electrolyte, which acts as a conductor for the oxygen anion. This is usually ytterbium-doped zirconium. The electrodes can be made of steel or ceramic depending on the operating temperatures and the desired electrolyte.                    or liquid, and is then an aqueous acid or base solution.                        
In the case of the electrolysis of water in an acid medium, the electrolyte is a solution of a strong acid, for example a solution of sulphuric acid (H2SO4) or of hydrochloric acid (HCl). However, managing concentrated acid electrolytes poses corrosion problems, and the technical solutions are very expensive.
Alkaline electrolysis is therefore the most widespread technology for the production of electrolytic hydrogen. In an alkaline electrolyzer, the electrolyte is an aqueous solution of potassium hydroxide (KOH). The ionic conduction is then ensured by the hydroxide (OH−) and potassium (K+) ions.
Current alkaline electrolysis systems operate at a voltage comprised between 1.7 and 2.1 V. The KOH solution has a concentration comprised between 25% and 35%. This method is currently implemented in conjunction with inexpensive sources of electricity (for example, hydraulic). Studies have also been conducted into particular cases, such as the operation of power stations during off-peak times or nuclear power stations dedicated to providing electricity to hydrogen production plants. Sources still in development, such as photovoltaic cells, are also proposed for providing electricity on a large scale for this method.
Today, nickel deposited on steel or solid nickel are the most commonly used electrode materials in industrial alkaline electrolysis systems.
The deposition technique used most often today in order to manufacture electrodes for the alkaline electrolysis of water is electrodeposition. This approach is of interest from an economical point of view as it limits the quantity of electrode material used. Moreover, it makes it possible to manufacture mechanically stable layers. The drawbacks of this technique are the limitation of the surface area developed by the electrode in contact with the electrolyte, leading to a deficiency in the associated performance and the complexity of the chemical compositions.
Different methods for manufacturing electrodes have already been proposed for cells for the decomposition of water by electrolysis, whether in an acid medium or in a base medium. There may be mentioned in particular:                the thermal decomposition of a salt of one or more precursor metals on a metallic support as described in the patent applications and patents FR 2581082, FR 2460343, FR 2547598, FR 2418280 and FR 2418281,        the electrodeposition of one or more metal salts on an electronic-conductor support such as described in the patent applications and patents FR 2385817, FR 2402477 and        the plasma deposition described in international application WO 2008067899 and the patent FR 2518583.        
Another example of the manufacture of electrodes for the electrolysis of water is described in the patent FR 2446870. The composite electrodes are composed of polytetrafluoroethylene (PTFE), carbon and noble metal oxides (Ru, Ir) by a multi-step method (grinding, heat treatment, pressing). The two main drawbacks of this method are its complexity and the choice of the materials used, which are not entirely suitable for this type of application. In fact, the polymer binder used (PTFE) is insulating and hydrophobic, which tends to reduce the performance of the electrodes. Moreover the carbon, although it has a good electrical conductivity, has a reduced resistance to corrosion under alkaline conditions.
The application JP 2012 065283 relates to an electrolysis method for the production of hydrogen gas which involves the reduction of water and the oxidation of ammonia gas, and a device for implementing said method. The ink is deposited on a transfer surface (PTFE membrane), then a transfer of the catalytic layer of PTFE to the surface of a metal foam is carried out, followed by assembly of the electrodes on each face of an anion membrane and impregnation of the anion membrane with KOH (or NaOH). This membrane acts as a solid electrolyte that makes it possible to transport the OH— ions. Optionally, a thin layer of carbon can be added between the catalytic layer and the metal foam. Thus the device which is described is a device for H2O/ammonia electrolysis based on the design of alkaline fuel cells, all the elements and assembling techniques of which it uses, ammonia replacing water in the anolyte compartment where nitrogen N2 is produced instead of O2. This device is a reverse alkaline fuel cell in which the hydroxide-conducting electrodes are impermeable to the gases (nitrogen and ammonia).
The application EP 0 622 861 relates to the manufacture of a membrane electrode assembly (MEA) for use as a fuel cell or for electrolysis for the synthesis of alkali metal hydroxides from the corresponding chlorides. These techniques use inks based mainly on Nafion®, which are deposited on the surface of ion (in this case proton) exchange membranes.
Efforts are therefore still to be made to increase the performance and the durability of the existing systems. To achieve this objective, novel electrode materials must be developed which will make it possible both to catalyse the electrochemical reactions in order to obtain a high current density for a low overpotential and to resist corrosion and mechanical stresses.
Recently, much research has focused on the development of novel electrocatalytic materials, in particular by structuring the conventional solid materials on the nanometric scale. Nanostructured materials are of interest in the field of catalysis and electrocatalysis because of their large developed surface area and the emergence of novel physical properties on this scale. In the different types of electrochemical reactions involved in the electrolysis of water in an alkaline medium, the use thereof is nevertheless difficult. This difficulty is, among other things, linked to the production of gas occurring at the two electrodes, which gives rise to stresses that are disadvantageous to the stability and mechanical integrity of the electrodes and which, as a result, leads to a loss of activity of the electrodes over time due to the loss of catalytic material.